The present invention relates to a method and system for reducing a power plant's sulfur emissions. In particular, the present invention relates to a method and system for recycling an increased sulfation capacity sorbent in a fluidized bed fossil-fuel combustor.
Electricity for residential, commercial and industrial use can be produced by combusting a fossil fuel in a furnace to generate high pressure steam. The steam can be allowed to expand in a turbine which will rotate and generate electrical power. By-products of burning a fossil fuel, such as coal, can include a combustion residue and flue gas. The combustion residue is largely fuel ash comprised of various inorganic substances, including silicon, aluminum, titanium, ferric, calcium and potassium oxides. The combustion residue can also include uncombusted fuel, and sorbent particles. The flue gas can contain large amounts of sulfur dioxide, unchecked release of which can have adverse environmental effects.
A sorbent, such as an alkaline earth oxide, can be used to remove significant amounts of the sulfur dioxide present in the flue gas by absorbing and retaining the sulfur dioxide in a solid sulfate form.
Fluidized bed combustion has distinct advantages for burning solid fuels and recovering energy to produce steam. In a circulating fluidized bed combustion system, fuel particles, typically crushed coal, are suspended in an upwardly flowing gas stream in a furnace. The fuel-gas combination can exhibit fluid-like properties. At an appropriate location, solids can be collected by a particle separator and circulated back to the furnace.
The solid fuel used to fire a fluidized-bed combustor can comprise non-fossil waste or fossil fuel derivatives. Typically, the solid fuel fed to a fluidized bed combustor is crushed coal mixed with a sulfur sorbent, such as limestone or dolomite particles. Use of a sorbent can permit 90% or more, depending upon the sulfur content of the fuel and the amount of sorbent added to the fluidized bed, of the sulfur dioxide released into the flue gas by fossil fuel combustion to be taken up by the sorbent.
Limestone, consisting largely of calcium carbonate, is a commonly used sulfur sorbent. Upon being fed into the fluidized bed of a combustor, the heat present can cause the limestone particles to undergo a calcination reaction to calcium oxide as follows: ##STR1##
After calcination and release of carbon dioxide, the sorbent particles become porous. The calcium oxide sorbent particles can absorb sulfur dioxide to form calcium sulfate: EQU CaO+SO.sub.2 +1/2O.sub.2 .fwdarw.CaSO.sub.4 ( 2)
The sorbent particles, with captured sulfur dioxide, remain in the combustion residue of the bed material.
Usually, only a fraction of the calcium oxide present in a typical sorbent particle reacts with and retains any sulfur dioxide. This is believed to be due to an initial rapid build up of calcium sulfate on the surface of the sorbent particle which blocks the pore structure of the sorbent particle. The interior bulk of the sorbent particle is thereby prevented from coming into contact with and absorbing sulfur dioxide.
Typically, a calcium to sulfur molar ratio of between about 1.5:1 to about 6:1 is required to capture about 90% of the sulfur released by combustion of fossil fuels in a fluidized bed reactor, depending on fuel and sorbent properties. Thus, about 40% to about 85% of the calcium oxide in a typical sorbent particle does not participate in any sulfur absorption.
Efforts have been made to increase sulfur sorbent utilization. Adding water to the combustion residue can hydrate the sorbent particles and increase the sulfation capacity of the sorbent particles by up to about 200%. When water is brought into contact with the combustion residue, hydration of the sorbent particles present in the combustion residue can take place as follows: EQU CaO+H.sub.2 O.fwdarw.Ca (OH).sub.2 ( 3)
Hydration can also cause the sorbent particle to swell and crack, thereby exposing additional surface area. Upon return of the combustion residue, including hydrated sorbent particles, to the fluidized bed of a fossil-fuel combustor, the sorbent particles can decompose to calcium oxide and water: ##STR2##
Significant additional amounts of calcium oxide are thereby exposed and made available to capture additional sulfur dioxide from the flue gas.
Because the spent sorbent particles are hydrated by contact with water, it is important to distribute the water as evenly as possible throughout the sorbent particles present in the combustion residue. Unfortunately, significant problems can arise when attempting to process the particulate combustion residue during and subsequent to treatment with water. Thus, a wet particulate matter tends to be cohesive and to lose its flow and fluidization properties. Additionally, combining sorbent with water can result in formation of a cement-like slurry. Furthermore, excess water can pool and interfere with combustion residue transport.
Previous attempts to address these problems by adding the water to the combustion residue in the form of a water/steam mixture have been unable to overcome the additional difficulties and restraints imposed due to the high temperature and pressure characteristics of steam.
Furthermore, although a mechanical or rotary hydrator can be used to reduce combustion residue aggregation as water is added to the combustion residue (to hydrate the sorbent particles present in the combustion residue), it is known that a mechanical hydrator can jam or otherwise malfunction due to the nature of the wet particulate matter present in the hydrator. Additionally, a mechanical hydrator can experience rapid abrasion of the parts in contact with the combustion residue and can therefore be expensive to operate and maintain.
A fluidized-bed hydrator can provide an even hydration fluid distribution to the spent sorbent particles present in the combustion residue, with a significant reduction of the aggregation and wear problems associated with use of a mechanical hydrator. U.S. Pat. No. 4,312,280, which is incorporated herein by reference in its entirety, discloses a fluidized bed hydrator for hydrating spent sorbent particles.
U.S. Pat. No. 4,313,280 discloses that subsequent to hydration, the combustion residue, including hydrated sorbent particles is returned to the combustor for further sulfur capture by the hydrated sorbent particles. Returning all the combustion residue to the combustor in this manner is inefficient because it is only the hydrated sorbent portion of the combustion residue for which there is any further use.
Unfortunately, there is no easy or practical way to separate hydrated sorbent particles from the rest of the combustion residue and to return only the hydrated sorbent particles to the fluidized bed combustor. Hence, the combustor ash load and the work of the ash handling equipment increases geometrically as each batch of combustion residue (with hydrated sorbent) is returned to the combustor from the hydrator. U.S. Pat. No. 4,312,280 addresses this problem by simply sending an unclassified portion of the combustion residue removed from the hydrator to waste. This is inefficient because a significant amount of the unclassified combustion residue sent to waste can include useful, hydrated sorbent particles. Hence, merely disposing an unclassified portion of the combustion residue to waste is inefficient and increases costs, as additional sorbent to replace that disposed of to waste must be obtained.
The practical inability to efficiently recycle sorbent particles in the fluidized bed of a fossil-fuel combustor can increase costs, reduce combustor life and create significant environmental hazards. For example, the cost of a sufficient amount of sorbent for a desired level of sulfur absorption is increased. Additionally, failure to efficiently recycle sorbent results in a larger amount of required sorbent. This in turn adds to the load of the combustion residue handling system, resulting in a greater auxiliary power outlay, more rapid equipment fatigue and failure and higher maintenance and replacement costs.
Furthermore, an excess of free lime sorbent particles in the combustor can result in increased levels of nitric oxide emission in the combustor flue gases. Excess free lime is also strongly alkaline and can therefore require that the combustion residue be neutralized for safe handling and to meet stringent disposal conditions and requirements imposed by various regulatory agencies.
Finally, an adverse environmental impact can result from the extensive quarrying for and disposal of the voluminous quantities of solid sorbent required when an efficient sorbent recycle is not carried out.
What is needed therefore is a method and system for efficiently recycling spent sulfur sorbent particles in a fluidized bed fossil-fuel combustor.